1. Field of the invention
The present invention relates to a correlation method and a correlator. More particularly, the present invention relates to a correlation method to adaptively operate in a time varying phase and a correlator thereof.
2. Description of the Related Art
A correlator is an electronic device to separate a weak information signal from noise by estimating a correlation function. Correlators such as a magnetic correlator and a cross correlator are known in the art.
Bit variations frequently occur in tags, specifically in low-cost tags that are essential to implement a ubiquitous computing technology. These variations can occur in a single tag according to the communications environment. However, variations also occur in tags needing no variation.
FIG. 1 shows types of a variation in a tag.
In FIG. 1, a positive phase variation of a1 is present in Tag 1, and a negative phase variation of a2 is present in Tag 2. In addition, there is also a bit variation T2-T1 between Tag 1 and Tag 2.
That is, there can be a variation of a signal phase in the tag because of reader backscatter communications where a symbol section changes due to the fast-changing communications environment. The variation can also be due to an unstable tag clock. Moreover, a symbol section can change between tags due to a different communications distance.
Conventional zero crossing detectors or cross-correlators can deal with the problems. However, the zero-crossing detector requires relatively large SNR which can not be frequently provided in backscatter communications.
It should be noted that the cross-correlator is based on the assumption that the symbol section of the received signals is essentially invariant. If the assumption of invariance in question is violated, the mentioned technique is likely to produce errors.